In many integrated circuit modules and in particular in SBC (Solder Ball Connection) modules, solder balls replace conventional connection pins. The substrate generally includes a plurality of layers which interconnect the chips to solder balls used as input/outputs. Such modules are routinely soldered to a printed circuit board by means of a matrix of conductive pads.
The overall reliability of the bonds created by the soldered ball process is adversely affected by defective balls. By way of example, if a ball has an insufficient height, no electrical connection exists between the ball and its corresponding pad on the circuit board.
In the past, several types of bonding inspection apparatus have been proposed. For instance, U.S. Pat. No. 4,688,939 describes an inspection apparatus of solder bumps by placing a chip carrier on a platform beneath a light ring which is coupled to a television camera. Light directed at an angle towards the sides of the chip carrier is reflected upwardly into the camera by the solder bumps. The output signal of the camera, which varies with the intensity of the light reflected from the bumps, is processed by a visual system to obtain a one-dimensional plot of the light intensity. The one-dimensional plot is analyzed automatically by the system to detect missing or excessive solder bumps on the chip carrier.
In a second illustration, U.S. Pat. No. 5,058,178 relates to an apparatus wherein defective or missing solder bumps of a chip carrier are detected by first illuminating the carrier with dark field illumination. The image of the surface is captured by a television camera. The image is processed to detect defects by first creating a window within the image about each group of solder bumps and creating a bounding box for each bump in each window. Each set of attributes includes the number, size and location of both the windows and the boxes within each window; and similarly, the dimensions, shape and brightness of the image within each box. The value of each attribute is compared to a reference value that represents the attribute when no defects are present. If the attribute differs by more than a predetermined tolerance, it indicates the presence of a defect.
Present state of the art inspection apparatus check each solder bump and verify whether the bump meets the requirements that determine the presence or the absence of a defect. None of the present apparatus inspect the complete array of bumps (or balls) to determine whether they are correctly placed with respect to each other within the array.